A known anti-skid control apparatus controls a braking force, which is generated at each wheel depending on slippage of each wheel, in order to avoid locking at the wheel. For example, an anti-skid control apparatus disclosed in JP2900542B is configured as follows. The anti-skid control apparatus first calculates a difference between pressure at a master cylinder and pressure at a wheel cylinder, and depending on the calculated difference between the pressure at the master cylinder and the pressure at the wheel cylinder, a pressure difference between an upper conduit of a pressure increasing valve and a lower conduit of the pressure increasing valve is linearly changed by outputting a control signal to the pressure increasing valve including a normally-open valve for controlling pressure increased at the wheel cylinder at each wheel, as a result, the anti-skid control apparatus smoothly increases the wheel cylinder pressure. Hereinafter, the master cylinder will be referred to as a M/C, and its pressure will be referred to as M/C pressure. On the other hand, the wheel cylinder will be referred to as a W/C, and its pressure will be referred to as W/C pressure.
Specifically, the pressure increasing valve functions as a linear valve for generating the pressure difference between the upper conduit and the lower conduit of the pressure increasing valve by linearly controlling a level of an electric current value supplied to the pressure increasing valve. More specifically, by controlling the level of the electric current supplied to the pressure increasing valve, a distance between a valve seat and a valve body of the pressure increasing valve is controlled, and a valve throttling effect generated between the valve body and the valve seat is changed in order to maintain the pressure difference depending on the valve throttling effect, as a result, the pressure increasing valve can function as a linear valve.
However, in the known configuration, during an ABS control, fluid force at the upper conduit of the pressure increasing valve may fluctuate while the W/C pressure is controlled so as to linearly change by means of the pressure increasing valve. For example, when the brake pedal is further and rapidly depressed, the fluid force may fluctuate. In this case, the W/C pressure may be rapidly increased because, for example, the valve body is distanced from the valve seat so that the pressure difference depending on the throttling effect between the valve body and the valve seat can not be maintained, as a result, a performance level for linearly controlling the W/C pressure may be decreased.
A need exists for an anti-skid control apparatus which is not susceptible to the drawback mentioned above.